1. Field of the Invention
The present invention relates to a checking method to be effected at the time of cavity preparation of full crown preparation that is the first stage of the restoration of tooth decay, especially posterior tooth decay and, more particularly, a method for objectively, readily and quantitatively measuring and reading the occulusal clearance between the pulpal cavity wall and the opposing tooth without recourse to the conventional method relying upon the sixth sense, thereby determining the proper occulusal clearance for the purpose of achieving the reliability and perpetuity of restoration and securing the preservation of tooth substances.
2. Statement of the Prior Art
Hitherto, the determination of the occulusal clearance has been made (1) by forming a guide groove within tooth substances and comparing the depth and thickness thereof with the length and diameter of a reference bar or point, or (2) allowing a softened paraffin wax plate to be bit upon and seeing through the thus bit portion under the illumination of light.
However, the first method comes short of expectation, and is unreliable, partly because unexpected regions are deficient in the amount of reduction. The second checking method is most widely used owing to its simplified operation and no need of using any special tool and material, but has the grave disadvantages, as mentioned in the following, which render it unreliable. Typically, those disadvantages are:
(A) Since the occulusion registered surface is in the negative form, it is hard to comprehend the whole thereof in three dimensions, when observing it outside of the mouth. Hence, it is very difficult to properly specify the regions to be restored, if any.
(B) At the time of occulusal closure, the impression itself undergoes deformation due to the deformation-due-to-flow, stress-on-removing and sticking of paraffin wax, which tends to give rise to a variation in the thickness of the wax of the associated region.
(C) The thickness of the occulusal clearance, i.e., the occulusal paraffin wax is presumed by comparison of relative transparency, and is thus very inaccurate, since it cannot be measured and determined on a quantitative basis.
The tooth substances should be properly reduced. In most cases, the posterior tooth decay (mainly due to dental cavities) of molars exposed to a strong mastication force is generally restored by a metal casting. In order to allow such a casting to stand up to the mastication force and fulfill its function over an extended period of time without any fall-off or damage and causing damage to the teeth per se, a variety of requirements are necessary, when preparing and reducing the teeth for receiving it. Of particular importance is the retention form. In the case of an internal casting (inlay), its retention is achieved by the surrounding cavity wall. In this case, it is required that the cavity floor be located as uniformly as possible and at a depth of about 0.5 to 1.0 mm in dentine. However, although conceptually feasible, to keep the cavity floor depth constant is actually more difficult than expected in clinical practice. In effect, such uncomfortable incidents as referred to below often occur by improper reduction.
I. Referring to the case where the clearance between the prepared surface and the opposite tooth is excessively insufficient, even the occulusal surface of the casting may be perforated at the time of occulusal adjustment during try-in of the casting. This is particularly disadvantageous to both patient and dentist in that the same procedure had to be commenced again from the re-preparation of teeth. Even when such perforation does not fortunately take place, the occulusal surface may locally become thin to an extreme degree. That thinned portion may eventually come to be perforated. Alternatively, stress concentration from mastication may take place in such a region, thus giving rise to the deformation of the casting. In some cases, the restoration casting may fall off due to the dislodgement of the luting material.
II. Referring to the case wherein the clearance between the prepared surface and the opposite tooth is excessive, excessive reduction of tooth substances causes damage to the dental pulp or weakening of the teeth, thus leading to a possibility of the tooth fracture.
As detailed above, the reasons for the "deficiency" or "excessiveness" of the clearance are considered to be due to lack of examination of its size, or owing to the fact that, although examined, any proper checking or determination of its size is almost impossible in the existing method.
III. Referring to the case where the clearance between the prepared surface and the opposite tooth is properly set, the thorough functions of the teeth nad restorations are expected to remain permanent. In most cases, however, the determination of the clearance relies upon the sixth sense, although properly achieved. Thus, proper clearence, even if it may be achieved, is far from reassuring, since its result differs from person to person and is poor in reproducibility.
With a view to eliminating the disadvantages of the aforesaid occulusal register methods with wax plates, Harumi Kurita proposed the method, as mentioned in the following (see "Inlay", edited by Kenji Marumori, pp. 67-71; published from ISHIYAKU SHUPPAN, Tokyo 1980). More exactly, Kurita prepared a scale by arranging a series of blue wax blocks having incrementally varying thicknesses. Then, Kurita observed a cavity depth in the thus obtained occulusion registered wax under the transmission of light, while comparing the cavity depth with the scale. However, since the occulusion registration is also in the negative form in this method, it still remains hard to specify the regions without making any modification thereto. Further, this method offers some problems hard to solve in that comparison in color with the scale is troublesome, considerable skill is required for proper determination, and so on.